Vibrations are ubiquitous in mechanical devices, machinery and vehicles, being generated for example by engines, motors, unbalanced rotors, roughness of the road, turbulences, etc. In most cases, these vibrations need to be damped or attenuated for an adequate performance of such devices. For instance, in case that an aircraft engine loses a blade and becomes unbalanced, vibrations in the aircraft need to be damped enough to let the pilot control the aircraft. In other few cases, controlled vibrations are purposely generated, such as relaxing coaches, powder transport, percussive drilling, etc.
Vibration transmission and damping has been therefore thoroughly studied in the field of mechanical engineering. Any rigid structure or mechanical system behaves as a vibration path or “circuit” for an oscillatory excitation being characterized by its mechanical impedance defined as the ratio between force and speed. There are three well-known kinds of elements: stiff elements, masses and dampers that can be combined to provide a total mechanical impedance for the system. Mechanical impedance depends on the frequency of the excitation. In the case of a viscous damper its mechanical impedance or damping ratio between the force exerted by the damper and the speed at which it is being elongated (or shortened) is constant for a broad range of frequencies.
A useful parameter to determine the resultant transmission of vibrations in a structure or bench is the transmissibility, defined as the ratio between the exerted force by such a structure or bench to its support divided by the force exerted by the source of vibrations on the structure or bench.
If a viscous damper with a large damping ratio is provided to a structure, then the greater the damping ratio is, the larger the energy dissipation and the lower the quality factor of the resonance. It is well known that damping reduces the maximum value of transmission—lightly shifting the frequency of resonance where such a maximum occurs—but increases the transmission coefficient in the high frequency regime. In other words, the use of a viscous damper reduces the transmissibility at frequencies around that of resonance but enhances the transmissibility in the high frequency regime. Additionally, the use of a viscous damper does not affect to the transmissibility in the very low frequency regime. For example, the well-known viscous oil dampers are used in automobiles in order to prevent resonance.
The simplest way to reduce vibrations transmission, for example from a rotating machine to the ground is to reduce the stiffness of the ground-connections. In this sense, elastic coupling is extensively used in industry and buildings. Typically a floating bench is used to support any vibrating machinery. In general, quite good vibration isolation can be obtained by appropriate design of these benches. However, when low frequency vibrations need to be suppressed, this approach becomes ineffective.
An alternative technology is dynamic vibration absorbers. They are designed to resonate absorbing part of the transmitted force. Adding damping to the absorber widens the bandwidth where the vibration isolation is effective. Again, these systems are only effective at the resonant frequency and they have worsening the behaviour in other regimes.
Additionally, there are particularly demanding environments where damping and control of vibrations remain unsolved problems. For instance, fuselage-mounted turbofans require a very efficient isolation system in order to assure a reduction in the transmission of the vibrations to the aircraft structure, but the high temperature of the engine (which can be as high as 650° C. close to the engine) prevents the use of most of systems like those based on viscous or viscoelastic materials.
Elastomeric materials with a high damping coefficient have been also used for example in the system disclosed in U.S. Pat. No. 4,199,128 A. However, they are limited by environmental conditions such as the operation temperature (typically operational temperature range from −55 to 70° C.) or the attack of engine fluids. Special silicones are able to work for a short time at even 150° C. Beyond that temperature, they present creep or drift and fatigue, exhibit nonlinear behaviour which is strongly dependent on frequency, temperature and loading conditions such as preloads and amplitude of motion. Alternatively, U.S. Pat. No. 6,491,290 B2 and U.S. Pat. No. 4,938,463 A present vibration chambers with fluid-filled chambers, but those systems present similar limitations to elastomer-based mounts in terms of temperature range and lifespan. Aerospace engines mounts—as for example that described in U.S. Pat. No. 4,875,655—hold the engine in the most severe flight conditions with high stiffness but no damping. This results in very reduced vibration isolation typically lower than 10 dB for low frequencies. There is a clear need of a damper suitable for this severe flight conditions.
Other currently used devices are active systems where an actuator (most frequently electromagnetic or piezoelectric) is driven to compensate the incoming vibration. These systems can be installed in parallel or in series with the connecting elements. However, the design of the feedback control loop is not an easy issue and required advanced signal control knowledge and power electronic systems. They need to be provided with a vibration sensor and an electronics processor that generates the compensating signal to command the active actuator. Typical drawbacks of electromechanical active systems include their high cost and weight and the requirement of control and sensing systems to operate with relatively high power consumption. Piezoelectric actuators are also used and able to provide higher forces. However, they are limited in the deformation they suffer. Therefore, this type of actuator frequently requires displacement amplification devices. Usually, they are stacked actuators, compliant mechanisms or hydraulic systems are preferred as amplifying systems to isolate “large-amplitude” vibrations. Each of these amplifying systems has their own limitations, including the temperature dependence of the strain (or deformation), the high voltages required, and the resulting non linearities and creep. Furthermore, these piezoelectric amplified actuators tend to be bulky, heavy and of complex design, limiting their applicability.
Semiactive-systems provide an alternative approach wherein only a parameter is changed as a function of the vibration conditions, saving power and requiring smaller control devices—for instance viscosity and damping coefficient using magnetorheological materials. Even though active and semiactive systems are more expensive and complex, they achieve vibration reductions of the order of 25 dB while passive systems typically achieve vibration isolation not better than 10-15 dB. Their main limitation is again the limited range of temperature for which sensors and active actuators can operate and survive.
An alternative approach is to use magnetic dampers based on eddy currents power loss in a conductor exposed to an alternating magnetic field, such as those disclosed by U.S. Pat. No. 4,517,505 A or U.S. Pat. No. 5,736,798 A. Arrangements of permanent magnets oscillating inside a conductive element are the most common design for this kind of devices. Some advantages of eddy current dampers are that they can operate from low temperatures up to above 300° C., they are fully passive and do not present contact between moving parts, minimizing wear and fatigue. In these devices, the kinematic energy in vibrating elements is dissipated as heat. However, power loss is proportional to the square of the speed of the oscillation what makes them useful only for relatively high frequencies but not very useful for damping of low frequency vibrations.